Diners Club and more
Diners Club appears to be the cheapest way to get lounge access around the world without tier cards or C/F tickets. In the UK, the Diners Club lounges are only at Glasgow, Manchester and Heathrow.
Note that there are circumstances in which a BA Gold card will not get you into a lounge. As a specific example, you cannot use the BA lounge here in Geneva when flying on a charter airline during the ski season. I suspect there are similar restrictions in other holiday destinations.
Other alliances are generally more restrictive with Silver tier cards. Silver cards tend to get you lounge access only in home countries. Lufthansa and Austrian have this restriction. Swiss does not. A few years ago in Houston on a C ticket, I discovered a Gold (Senator) card was needed for access to the lounge. Don't know whether that is still the case.
Momo